The effects of DDT, dieldrin, carbaryl, parathion, piperonyl butoxide and Aroclor 1254®, a polychlorinated biphenyl containing 54% chlorine, on growth and photo-synthesis in the freshwater algaChlorella pyrenoidosaChick were measured. Inhibition was most easily demonstrated by varying the concentratIon of the alga and keeping the chemical concentration constant. Aroclor 1254 was the most active inhibitor of growth and photosynthesis inC. pyrenoidosa. Piperonyl butoxide, an insecticide synergist, and parathion were also strong inhibitors of growth and photosynthesis while DDT was less active. Dieldrin and carbaryl were least potent as inhibitors.